Fixes issue where computing storage size for a number would take too long.

Fixes #4673.

libsolidity/ast/Types.cpp

@@ -1263,7 +1263,8 @@ shared_ptr<FixedPointType const> RationalNumberType::fixedPointType() const
 		return shared_ptr<FixedPointType const>();
 	// This means we round towards zero for positive and negative values.
 	bigint v = value.numerator() / value.denominator();
-	if (negative)
+
+	if (negative && v != 0)
 		// modify value to satisfy bit requirements for negative numbers:
 		// add one bit for sign and decrement because negative numbers can be larger
 		v = (v - 1) << 1;

libsolidity/ast/Types.h

@@ -446,7 +446,8 @@ public:
 
 	/// @returns the smallest integer type that can hold the value or an empty pointer if not possible.
 	std::shared_ptr<IntegerType const> integerType() const;
-	/// @returns the smallest fixed type that can  hold the value or incurs the least precision loss.
+	/// @returns the smallest fixed type that can  hold the value or incurs the least precision loss,
+	/// unless the value was truncated, then a suitable type will be chosen to indicate such event.
 	/// If the integer part does not fit, returns an empty pointer.
 	std::shared_ptr<FixedPointType const> fixedPointType() const;